1. Field of the Invention
The present invention relates to an improvement of a radio-frequency identification tag communication device capable of radio communication with radio-frequency identification tags for writing and reading information on and from the radio-frequency identification tags.
2. Discussion of the Related Art
There is known an RFID (Radio-Frequency Identification) communication system wherein a radio-frequency identification tag communication device (interrogator) reads out information, in a non-contact fashion, from small radio-frequency identification tags (transponders) on which desired information is written. In this RFID communication system, the radio-frequency identification tag communication device is capable of reading out the information from the radio-frequency identification tags, even where the radio-frequency identification tags are contaminated or located at positions invisible from the radio-frequency identification tag communication device. For this reason, the RFID communication system is expected to be used in various fields, such as administration and inspection of articles of commodity.
There are generally well known array antenna techniques available in a radio-frequency identification tag communication device which has a plurality of antenna elements and which is arranged to effect radio communication through those antenna elements. Examples of such array antenna techniques include a phased-array control of the phase of a signal corresponding to each of the antenna elements, and an adaptive-array control of the phase and amplitude of the signal corresponding to each antenna element. Patent Document 1 discloses an example of an adaptive-array antenna controller arranged to change an antenna control coefficient according to the position (direction) of an object tag, for thereby increasing a maximum distance to the object tag with which the communication device can communicate.
Patent Document 1: JP-11-251996 A
However, the plurality of radio-frequency identification tags with which the radio-frequency identification tag communication device communicates are usually located at mutually distant positions, and reflected waves transmitted from those tags have extremely low intensities, so that the maximum distance of communication with the tags according to the conventional array antenna techniques is limited, and the communication device has a risk of failure of communication with the desired radio-frequency identification tags. Accordingly, there has been a need of developing a radio-frequency identification tag communication device having an increased maximum distance of communication with the radio-frequency identification tags.